The present invention relates to an overwritable magneto-optical recording medium and recording method for use in recorders.
The magneto-optical recording, as well known, is the optical recording capable of writing, reading and erasing information and becomes the object of public attention in that it is capable of high-density recording which is very superior to the conventional magnetic recording. In addition, recently, a new magneto-optical recording method called overwrite has been proposed in which new information is written on a magneto-optical disk having information previously written, while the previously written information is being erased. This overwritable magneto-optical recording system can be roughly divided into a magnetic field modulation method and a light intensity modulation method.
As compared with the magnetic field modulation method, the light intensity modulation method is advantageous because the latter method can make high-speed modulation and high-speed recording and use a both-side recordable disk having two magneto-optical recording media respectively bonded to the opposite sides. The magneto-optical recording medium for realizing this light intensity modulation method is already known from the principle point of view as disclosed in, for example, Japanese Patent Laid-open Gazette No. 62-175948. That is, the magnetic layer used in the recording medium of this kind is an exchange-coupled double layer film having magnetic coupling.
The overwrite will be briefly described with reference to drawings. FIGS. 2 to 5 are diagrams useful for explaining the light intensity modulation method using the known exchange-coupled double layer film as the magnetic layer. The magnetic film for use in recording is a double layer film that is formed of a recording layer 15 and a memory layer 4.
FIG. 2 shows the relation between the temperature and the magnetic field and coercive force. At room temperature T.sub.r, a coercive force 8 of the memory layer 4 is larger than a coercive force 9 of the recording layer 15, and a Curie temperature T.sub.9 of the recording layer 15 is higher than a Curie temperature T.sub.8 of the memory layer 4.
Upon recording, as shown in FIG. 3, the recording layer 15 is magnetized in the same direction as an initializing field 13 by an initializing magnet 17 as indicated at 18. The initializing magnet 17 is normally fixed to the disc drive so as to oppose the disk. The initializing magnet 17 is formed of a magnet or an electric magnet, and it has a constant intensity and produces a magnetic field in a constant direction. The magnitude of this initializing magnetic field 13 is larger than the coercive force 9 of the recording layer 15 and lower than the coercive force 8 of the memory layer 4 at room temperature T.sub.r as shown in FIG. 2. Thus, even when the initializing magnetic field 13 is applied to the memory layer 4, the direction of the magnetization 19 of the memory layer 4 is not changed.
Upon recording of information, the intensity of laser light is modulated by digital information of "1"s and "0"s being recorded under a constant-intensity bias field 14 shown in FIG. 2 so as to vary between a high laser power level (hereinafter, referred to as P.sub.H) 23 and a low laser power level (hereinafter, referred to as P.sub.L) 24 as shown in FIG. 6. The constant-intensity bias field 14 which is applied upon recording is different in both field strength and direction from the initializing field 13. This constant-intensity bias field 14 is produced in the constant direction from a magnet 22 which is fixed to the disk drive to oppose the disk as is the initializing magnet 17.
As illustrated in FIG. 4A or FIG. 5A, laser light 20 is focused into a predetermined spot diameter by a focus lens 21, and irradiated on the magnetic film. When the laser light intensity is P.sub.H, the region irradiated with the laser light 20 is heated near to the Curie temperature T.sub.9 of the recording layer 15 as shown in FIG. 2, and hence the temperature of the memory layer 4 already exceeds the Curie point T.sub.8. Thus, the magnetization 19 of the memory layer 4 is extinguished as shown in FIG. 4A. On the other hand, the magnetization 18 of the recording layer 15 in the region at this temperature is in the same direction as that of the bias field 14 (in the opposite direction to the initializing field 13) since the bias field 14 from the magnet 22 is already larger than the coercive force 9 of the recording layer 15 as will be obvious from FIG. 2. When the laser light 20 is stopped from the irradiation, allowing the magnetic film to cool, the magnetization 19 of the memory layer 4 occurs in the same direction as the bias field 14, thus recording information.
When the laser light intensity is P.sub.L, the temperature of the region irradiated with the laser light 20 is near the Curie temperature T.sub.8 of the memory layer 4 as shown in FIG. 5A. Therefore, the coercive force 9 of the recording layer 15 is larger than the bias field 14 as shown in FIG. 2, and thus the magnetization 18 is not reversed. When the laser light is stopped from the irradiation, allowing the magnetic film to cool, the magnetization 19 of the memory layer 4 is also in the opposite direction to the bias field 14 as shown in FIG. 5B because it has a large exchange coupling force with the magnetization 18 of the recording layer 15 than the bias field 14.
As described above, the light intensity modulation method using the exchange coupling dubble film makes overwriting by modulating the laser light intensity to vary between the high level (recording) and the low level (erasing). In addition, upon reading-out of information, light (read-out) of which the level is smaller than the low level light is irradiated to the film. Thus, the laser light intensity is changed in three steps of different levels for recording, erasing and reading-out. Moreover, in order to cope with various disturbances such as the reduction of laser efficiency associated with the change of ambient temperature and laser output and the contamination of optical parts and to record, erase and read-out data without destroying data, it is necessary to take enough margin for these power levels.
The above conventional novel technique, however, has no consideration for assuring enough margin for the three power levels. Thus, this conventional technique cannot cope with various disturbance and has the problem that upon recording, erasing and reading-out the reliability of information is reduced.
In Proc. Int. Symp. on Optical Memory, 1989, a diagram (FIG. 3) of laser power margin is shown for the irradiation of light of f=4.93 MHz (modulated light).
In this paper, however, there is no description and suggestion about this invention. According to this invention, when laser light of a constant intensity with no modulation is irradiated on the continuously recorded stripe-shaped magnetic domain for which a widest power margin is required, making overwriting, the required power margin is found, and the film structure is set to have more than this margin or the light irradiation conditions are controlled for more than this margin, thereby making it possible to overwrite not only on the spot domain but also on any domain.
Moreover, as described in the summaries, 10PF-6 of the fourteenth scientific lecture of the Magnetic Society of Japan, the gradient of the plot of the transfer capability (corresponding to a proportional constant K in the equation, Ht.sub.1 =-K.multidot.T+A (A is a constant) in this invention: this equation is the relation of the switching field Ht.sub.1 of the memory layer relative to temperature T at an erase-starting temperature at which the recorded information starts to be erased when the medium temperature is increased) at a high temperature is only 50, maximum which is out of the range of 50&lt;K&lt;220 according to this invention. Also, in this paper, there is no description and suggestion of the idea of the invention in which the film structure is set or the light irradiation conditions are controlled, so that 50&lt;K&lt;220 can be satisfied.